docs: def availability(self, dcid, params=None):
          ''' /v1/regions/availability
          GET - public
          Retrieve a list of the VPSPLANIDs currently available
          in this location. If your account has special plans available,
          you will need to pass your api_key in in order to see them.
          For all other accounts, the API key is not optional.
  
          Link: https://www.vultr.com/api/#regions_region_available
          '''
          params = update_params(params, {'DCID': dcid})
          return self.request('/v1/regions/availability', params, 'GET')